1. Field of the Invention
The present invention relates to a method utilized in a wireless communication and communication device thereof, and more particularly, to a method for improving a buffer status triggering mechanism in a wireless communication system and communication device thereof.
2. Description of the Prior Art
As today's applications for electronic systems grow at ever-increasing rates, the demand for better communications performance is never ceasing. Standards for various technologies such as the 3rd Generation Partnership Project (3GPP) High-Speed Packet Access (HSPA) and Long Term Evolution (LTE) work towards creating more efficient communication systems.
For example, dynamic resource allocation for uplink (UL) transmissions has been introduced in the a medium access control (MAC) layer standard, for communicating between a user equipment (UE) such as a mobile station or a mobile handset, and an Evolved Universal Terrestrial Radio Access Network (E-UTRAN) including evolved base stations (eNBs).
Dynamic resource allocation utilizes radio resources more efficiently, where radio resources are allocated only when the UE has data to transmit. The E-UTRAN schedules UE transmission based on UE buffer status. The UE buffer status reporting has a great impact on efficiency of overall radio source usage arranged by E-UTRAN. Reporting more data volume than actual data volume in the buffer makes unnecessary uplink transmission and consumes UE power. Reporting less data volume than actual data volume in the buffer makes the UE inevitably request for more resources to be allocated and wait for the uplink resource allocation, and this causes a delay before the UE data can be transmitted, slowing down the overall communication between the UE and the E-UTRAN.
In striving for faster and better communications and efficient usage of radio resource in our wireless communications systems, the unnecessary uplink resource allocation and uplink transmission delay should be alleviated.
An RRC (Radio Resource Control) connection re-establishment procedure can be initiated by the UE to re-establish the RRC connection, which involves the resumption of SRB1 (Signaling Radio Bearer 1) and the re-activation of security. The UE shall only initiate the procedure when security has been activated. The UE initiates the procedure when one of the following conditions is met: (1) a radio link failure is detected; (2) upon handover failure; (3) the maximum number of retransmissions has been reached; (4) upon integrity failure indication from lower layers; (5) upon an RRC connection reconfiguration failure.
Message exchange of the RRC connection re-establishment procedure between the UE and the EUTRAN is described below. The UE sends an RRC connection re-establishment request message to initiate the RRC connection re-establishment procedure. The EUTRAN transmits an RRC connection re-establishment message to accept the request. Finally, the UE responds with an RRC connection re-establishment complete message to notify the EUTRAN of the completion of the RRC connection re-establishment procedure.
Through the RRC connection re-establishment procedure, the SRB1 can be resumed. For resumption of other radio bearers, such as SRB2 or DRBs (data radio bearers), an RRC connection re-configuration procedure is initiated following RRC connection re-establishment procedure. The RRC connection re-configuration procedure is initiated by the EUTRAN with an RRC connection re-configuration message, and an RRC connection re-configuration complete message is sent by the UE to complete the RRC connection re-configuration procedure.
A buffer status reporting procedure is used for the UE to provide the serving eNB with information about the amount of data in the UL buffers of the UE. A Buffer Status Report (BSR) is defined as a MAC (Medium Access Control) Control Element and shall be triggered if any of the following conditions occur:
(1) UL data becomes available for transmission in the RLC entity or in the PDCP entity and the data belongs to a logical channel with higher priority than those for which data already existed in the UE transmission buffer, in which case the BSR is referred below to as “Regular BSR”;
(2) UL resources are allocated and number of padding bits is equal to or larger than the size of the Buffer Status Report MAC control element, in which case the BSR is referred below to as “Padding BSR”;
(3) a serving cell change occurs, in which case the BSR is referred below to as “Regular BSR”; and
(4) a PERIODIC BSR TIMER expires, in which case the BSR is referred below to as “Periodic BSR”.
As can be seen from the above, the BSR includes three types: Regular, Periodic and Padding types. In addition, the regular BSR is able to trigger a Scheduling Request (SR) for UL (Uplink) resources.
Furthermore, according to the buffer status reporting procedure, a PERIODIC BSR TIMER is started when the regular BSR is sent. When the PERIODIC BSR TIMER expires, the periodic BSR is sent if there is UL resources for new transmission in this TTI (Transmission Time Interval). Otherwise, the periodic BSR is not sent. In this BSR triggering behavior, the regular BSR triggering condition affects operation of the PERIODIC BSR TIMER and thus is very important. As can be seen from the above, the regular BSR can be triggered by the conditions (1) and (3). The condition (1) is used for all necessary situations other than handover, whereas the condition (3) is only applied to handover.
According to the condition (1), “the data belongs to a logical channel with higher priority than those for which data already existed in the UE transmission buffer” indicates that the UL data available for transmission needs to be compared with any data that has existed in the UE transmission buffer for logical channel priority. However, condition (1) causes an issue that no more BSR is triggered but there is still data available for transmission, which needs to be reported, in the UE transmission buffer.
In addition, “UL data becomes available for transmission in the RLC entity or in the PDCP entity” quoted from the condition (1) can cause an issue that the UL data available for transmission is reported twice, resulting in space waste of the BSR.
Four issues are described below with the current BSR triggering mechanism.
Issue 1 is described here. A UE RLC AM (Acknowledged Mode) entity associated with a logical channel receives a SDU (Service Data Unit) for transmission. Since the SDU for UL becomes available for transmission, a regular BSR is triggered according the abovementioned condition (1). Assume that no UL resources are allocated for new transmission for a TTI where the regular BSR is triggered. As a consequence, an SR is triggered and then sent. The eNB receives the SR and allocates a UL grant providing sufficient UL resources for SDU transmission. The UE receives the UL grant. The UE RLC AM entity constructs an RLC data PDU including this SDU. A poll bit is set in the RLC data PDU to request the eNB for ACK/NACK reporting, and thereby a POLL RETRANSMIT TIMER is started. The UE RLC AM entity submits the RLC data PDU to the MAC layer. The UE MAC constructs a MAC PDU including the RLC data PDU, also seen as a MAC SDU, and requests a PHY (Physical) layer to transmit. The MAC PDU transmission is failed. After a while, the POLL RETRANSMIT TIMER expires, indicating that the RLC data PDU in the transmission buffer needs to be retransmitted. However, according to the condition (1), no BSR is triggered because the RLC data PDU is already in the UE transmission buffer and furthermore, the RLC data PDU impossibly has higher priority than itself. As a result, the uplink data transmission is blocked since no BSR is triggered for the opportunity of UL resource allocating request (i.e. the SR).
Issue 2 is described here. A UE RLC AM entity associated with a logical channel X receives a SDU for transmission. According the condition (1), a regular BSR is triggered. When no UL resources allocated for new transmission for a TTI where the regular BSR is triggered, an SR is sent. The eNB receives the SR and thereby allocates a UL grant providing sufficient resources for SDU transmission with segmentation. After receiving the UL grant, the UE RLC AM entity constructs an RLC data PDU including this SDU. In addition, a poll bit is set in the RLC data PDU, and a POLL RETRANSMIT TIMER is started. The UE RLC AM entity submits the RLC data PDU to the MAC layer. The UE MAC constructs a MAC PDU and requests the PHY layer to transmit. Unfortunately the MAC PDU transmission is failed. Before the POLL RETRANSMIT TIMER expires, another UE RLC AM entity associated with a logical channel Y receives a SDU for transmission. Priority of the logical channel Y is lower than priority of the logical channel X. According to the condition (1), no BSR is triggered for the SDU of the logical channel Y because the RLC data PDU of the logical channel X has already been in the transmission buffer and the logical channel Y priority has lower priority than the logical channel X. As a result, the uplink data transmission for the SDU of the logical channel Y is blocked.
Issue 3 is described here. A DRB (Data Radio Bearer) is configured with the PDCP entity and the RLC entity to transmit IP (Internet Protocol) packets. Assume that a PDCP buffer and an RLC buffer are both empty. The PDCP entity receives an IP packet (i.e. a PDCP SDU) from an IP layer. In this situation, the condition (1) is met because the PDCP data becomes available for transmission. As a result, a regular BSR is triggered. Assuming that UL resources are available, the MAC layer sends the regular BSR immediately. After header compression and ciphering of the PDCP SDU, the PDCP entity generates a PDCP PDU and submits the PDCP PDU to the RLC entity. In this situation, the condition (1) is met again because the RLC data becomes available for transmission. Thus, another regular BSR is triggered. Assuming that UL resources are available, the MAC layer sends a regular BSR again. As can be seen from the above, the regular BSR is triggered and sent twice for the same IP data, one time for the PDPC layer and the other time for the RLC layer. This increases overhead due to an unnecessary buffer status reporting.
Issue 4 is described here. A UE has two logical channels, one for SRB1 and the other for a DRB, and initiates an RRC connection re-establishment procedure due to a radio link failure. When the UE receives an RRC connection re-establishment message, the UE shall send an RRC connection re-establishment complete message corresponding to SRB1. The UE has no UL grant to transmit the RRC connection re-establishment complete message, and the logical channel priority of SRB1 is lower than or equal to the logical channel priority of the DRB. Besides, the PDCP buffer includes data of the DRB that has been transmitted by RLC before but has not been acknowledged or negatively acknowledged before the radio link failure. In this case, no BSR is triggered according to the condition (1). As a result, the RRC connection re-establishment complete message cannot be transmitted. This causes failure of the RRC connection re-establishment procedure.
Issue 5 is described here. At RRC connection re-establishment triggered by a radio link failure, SRB 1 is resumed by an RRC connection re-establishment procedure, and other radio bearers, such as SRB2 and DRBs, are resumed by a following RRC connection reconfiguration procedure. If a regular BSR is triggered before any available radio bearers other than SRB 1 are resumed, the regular BSR includes data available for transmission for all available radio bearers including SRB1 when the PDCP buffer includes data available for transmission related to the available radio bearers. However, it may be useless to provide buffer status of the radio bearers other than SRB1 with the regular BSR since the radio bearers are still suspended. The resources, which are allocated by the eNB for the suspended radio bearers, are wasted. Furthermore, in certain situations (e.g. a session ended by an application), some of the suspended radio bearers are possibly released by the following RRC connection reconfiguration procedure instead of being resumed. In this case, it is useless to include buffer status for the released radio bearers in the regular BSR.